Field of the Invention
The invention relates to a functional monitoring system, in particular access control system, having a central transmitting and receiving station and a plurality of transponders which can communicate bidirectionally with the transmitting and receiving station.
European patent EP 0 440 974 B1 and U.S. Pat. No. 5,552,641 disclose such functional monitoring systems in the form of motor vehicle access control systems. There, a central transmitting and receiving station arranged in or on the motor vehicle transmits an interrogation code signal (question code) for example after the actuation of a door actuating button. The signal is answered by a portable transponder located in radio range. The motor vehicle access control system described in U.S. Pat. No. 5,552,641 may be configured with multichannel capability for security reasons.
If, for such an access control system, a plurality of portable transponders are issued for use by a number of persons, the functional monitoring system, in the case of desired functional control, should not only be able to identify whether one or more transponders are located in the detection range of the central transmitting and receiving station, but also be able to detect which transponder or transponders is or are involved. This is true particularly if the individual transponders are assigned different functional scopes (by way of example, one transponder may be allowed only to unlock the central locking system, while another transponder may, in addition to actuating the central locking system, also if disconnect the immobilizer device and thus enable the motor vehicle for a driving mode).
The functional monitoring system must therefore check whether, and which, transponders are located in the detection range. At the same time, it must be ensured that a plurality of authorized transponders do not mutually influence one another during the transmission of their response code signals (answer code), such that the system can no longer check the correctness of the response code signals and hence the desired function is also not enabled. The communication thus needs to be safeguarded against collisions. One possible approach for solving this problem consists in allocating different identifiers, e.g. numbers, to the transponders and directing the transponder search in each case only at one transponder, that is to say transmitting a dedicated interrogation code signal for each transponder (e.g. with an address which designates only this one transponder). The transponders belonging to a system are then successively interrogated as to their presence by multiple repetition of the transmission of the interrogation code signal, which is progressively altered in each case, and waiting for an response code signal. However, this procedure takes up an appreciable amount of time, with the result that an undesirable time delay may possibly occur between a user requesting a specific function and the function that is actually being carried out.